MgAl_(2)O_(4)包覆对钨铝复合氧化物水热稳定性的影响

Effects of magnesium-aluminate spinel coating on hydrothermal stability of tungsten-aluminum composite oxides

钨铝复合氧化物WO_(x)/Al_(2)O_(3)(WAl)在含有水或者反应过程中有水生成的条件下不稳定,其中γAl_(2)O_(3)容易发生水合而导致相变,造成比表面积、孔容、孔径减小而使催化剂失活。采用浸渍焙烧法在WAI表面制备不同含量的镁铝尖晶石(MgAl_(2)O_(4)),并利用水热釜对MgAl_(2)O_(4)包覆前后WAl的水热稳定性进行考察。采用粉末X线衍射(XRD)、N_(2)吸附脱附、扫描电子显微镜(SEM)及衰减全反射红外光谱(ATRIR)等对水热前后的样品进行表征。结果表明:所有的样品在水热初期比表面积增加,孔容、孔径减小,并且WAl样品发生了相变。随着水热时间达到500 h后,WAl中的γAl_(2)O_(3)已完全转变为片状结构的勃姆石(AlOOH),比表面积、孔容、孔径急剧下降。而包覆MgAl_(2)O_(4)后,WAl的水热稳定性明显提高,当MgAl_(2)O_(4)的质量分数≥12%时,并未观察到明显的WAl相,表明此时WAl的水热稳定性得到有效的提高。

Tungsten-aluminum composite oxides WO_(x)/Al_(2)O_(3)(WAl)are unstable under the condition of containing water or generating water in the reaction,in whichγ-Al_(2)O_(3) is easy to hydrate and leading to phase transformation,resulting in the reduction of specific surface area,pore volume and pore diameter,and therefore the catalyst will be inactivated.WAl support coated with different content of magnesium-aluminate spinel(MgAl_(2)O_(4))was prepared by impregnation-calcination method,and the hydrothermal stability of the coated WAl support was investigated by hydrothermal kettle.The samples before and after hydrothermal treatment were characterized by powder X-ray diffraction(XRD),N_(2) adsorption-desorption,scanning electron microscopy(SEM)and attenuated total reflection infrared spectroscopy(ATR-IR).Results showed that the specific surface area increased,pore volume and pore diameter decreased at the initial stage of hydrothermal treatment of all samples,and the WAl support had evidently phase transformation.With the hydrothermal treatment time reaching 500 h,γ-Al_(2)O_(3) in the WAl sample had completely transformed into the plate-like structure of boehmite(AlOOH),the specific surface area,total pore volume and average pore diameter had sharply declined.The hydrothermal stability of WAl support greatly improved after coating MgAl_(2)O_(4).When the MgAl_(2)O_(4)mass fraction was more than 12%,no phase transition was observed,which indicated that the water resistance of WAl support was best.